The in wall units of that size, as mentioned, will pull out of the case after removing the faceplate which usually has a couple three screws and unhooks at the top.
At that large BTU size a replacement could cost you $1,000.
AND don't assume you can slide new "guts" in to the old case. The new ones can be a different size and then you have installation, construction type cost to replace the case in the wall .
So exploring a repair is well worth it. Even a new compressor might be half the cost of a replacement the same size. You have to weigh that against the age and general condition.
But it very easily could be a "relatively" inexpensive repair, like a compressor relay, start delay, thermostat, overload, high temp safety, start cap, or the like that has failed.
And you can also save yourself a trip charge (saves about $95) by pulling it out and taking it in to a shop. But those big units aren't light. You might want two people to carry it.
And "in shop" rates for labor are a little lower than "in home" rates. So you might want to ask about that if you call around for a repair.
Plus if you go to replace it, you need to dispose of the old one which requires removing the freon and there is usually a cost. So if it is already in the shop and you decline the repair they can take care of the disposal.
That imo, is the best route, at least get an estimate in case it is a simpler repair.