The common causes are mundane. A budget was frozen, a decision maker went on holiday, a competing priority arrived, or the person you were dealing with is waiting on somebody who has not responded. None of those are about you, and all of them produce the same silence that a genuinely unhappy client produces, which is why guessing is unproductive.
Send a short message rather than a long one. Two sentences asking whether the timing still works, with an offer to pause, produces a reply far more often than a detailed status update or a request for feedback. The reason is that a short message is easy to answer honestly, and somebody who has been avoiding you because they have no answer can now say so without composing an explanation.
Offer the pause explicitly, because that is usually what they want and cannot ask for. A client who has run into a problem on their end is frequently embarrassed rather than annoyed, and giving them a way to defer without it being a failure resolves the situation immediately. It also converts an indefinite silence into a known date, which is what you actually need.
Escalate the channel rather than the frequency. Three emails to somebody not reading email produces nothing except a record of you chasing. One phone call, or a message on whatever channel they actually respond to, resolves in minutes what a fortnight of emails did not. If you have no alternative channel, that is worth fixing at the start of the next engagement.
Set a limit on how long you hold the work open, and state it. Something like keeping the slot until a specific date, after which the project moves to the back of the queue, is honest and reasonable rather than pressuring. It protects your capacity, which is the real cost of an indefinite pause, and it frequently prompts a decision.
Protect yourself commercially while the situation is unresolved. Invoice for the work completed to date rather than waiting for a milestone that may not arrive. Deposits and staged payments exist precisely for this, and a project that stalls after a deposit and a first stage is an inconvenience rather than a loss.
Document where things stand while it is fresh. If the project resumes in three months, the version of your notes written today is considerably more useful than the reconstruction you would attempt then, and it also protects you if the client returns with a different memory of what was agreed.
Then look at the timing across several projects, because a pattern is informative. If clients frequently go quiet at the same stage, that stage is doing something: it may be where the cost becomes real, where a decision requires somebody who was never involved, or where you asked for something they find difficult to produce. That is fixable, and it is invisible from any single instance.
Keep the tone neutral rather than concerned, because a message expressing worry invites reassurance rather than an answer. Asking whether the timing still works is easy to reply to honestly. Asking whether everything is alright puts the client in the position of managing your feelings about their silence, which is one more reason not to respond.
Log the date you last heard from them somewhere you will see it, because the gap is easy to underestimate. What feels like a few days is frequently three weeks, and the response is different at each point. A note against the project turns a vague sense that it has been a while into a fact you can act on.