You open your site and instead of the usual page you get an account suspended notice. It is one of the biggest scares a host can give you, and the first reaction is usually to assume it is a provider error. Most of the time it is not: a suspension has a concrete cause, and there were almost always warning signs before it.
The real reasons, not just missed payment
The first one everybody thinks of is not paying the renewal, and it is indeed the most common. But there are other reasons, just as frequent, that surprise people more because they involve no carelessness with money at all.
Excess resource usage. A shared plan has caps on processor, memory and simultaneous processes. If an account sustains consumption above what it is entitled to, it affects the other accounts on the same server, and the provider suspends it to protect the rest. It is not an arbitrary punishment: it is how the service stays even for everyone.
Sending mass spam. This could be a badly configured form a bot is using to send spam, or a compromised email account. Mail providers worldwide rate every server's reputation, and one site sending spam can wreck mail delivery for everyone else on that server. That is why the response is usually immediate.
Detected malware. A site that is infected and also distributing that malware to its visitors, or that the server itself detects sending malicious traffic, gets suspended to stop the damage while it is cleaned.
Disallowed use. Hosting content that violates the terms of service, or using the hosting for something other than what the plan is meant for (for example, running heavy processes constantly on a shared plan meant for websites).
Expired or false contact details. If the provider cannot verify who the account holder is, some apply a preventive suspension until the information is updated.
The signs that almost always come first
Except in cases of obvious fraud, a suspension is rarely instant with no warning at all. It is worth knowing the signs so you can act before it reaches that point.
If it is resources: the site starts running slower overall, not just at peaks, and the control panel may show elevated usage alerts if you have that section enabled.
If it is mail: you start receiving bounces for messages you never sent, or notice sending activity in the mail account's statistics that you do not recognise.
If it is payment: an upcoming renewal notice arrives, then an actual expiry notice, and only then the suspension. The gap between the first notice and the suspension is usually several days.
If it is malware: your hosting's antivirus, if you have one included, reports infected files before the situation escalates to a suspension.
The common pattern: there is almost always a window to act. The problem is that many of those notices arrive at a mailbox nobody checks often.
How to avoid it
Keep the account's contact email current, and check it with some regularity. It is the channel through which almost every prior warning arrives.
Turn on auto-renewal and check every so often that the card attached is still valid. Most payment-related suspensions are not because someone chose not to pay, but because auto-renewal failed silently.
Keep WordPress, plugins and the theme updated. It is the most effective defence against malware, which is the hardest cause to prevent once it has already gotten in.
Check your contact forms once in a while, especially if you notice odd spam messages in the replies: it can be a sign a bot is using it to relay mass mail through your server.
If you have a project that is growing, do not wait until the plan runs short. Moving to a higher plan before hitting the limit is far simpler than doing it after a suspension.
What to do if your account has already been suspended
The first thing is not to panic and to contact support right away, preferably through the fastest channel available (chat or phone, if the provider offers one). The team can tell you the exact reason, which is not always obvious from the outside.
If it is payment, settling the account usually reactivates the service within minutes. If it is resources or malware, you will need to resolve the cause (optimise the site, clean the infection) before it can be reactivated, because reactivating it without fixing the underlying problem ends in the same suspension shortly after.
While the account stays suspended, the site is not visible to anyone, so the sooner it gets resolved, the less impact it has on visits and rankings. That is why it is worth keeping the support phone number or chat handy in advance, not something you have to look up only after it has already happened.
What to check once it is reactivated
Once it is back up, do not consider the matter closed without checking a bit further. If it was resources, watch usage closely over the following days: if it starts creeping back toward the limit, it is better to get ahead of the next suspension than to wait for it again. If it was malware, run a full scan and change every password tied to the account, not just the one you suspect was compromised.
And if the cause was a form being abused for spam, confirm it is protected before someone exploits it again: a suspension solved without closing the door the problem came through tends to repeat itself.