Our shower used to have great water pressure, as does all the taps/faucets in the house, but recently it just went down to a trickle. Some initial googling suggested it was most likely a blockage in the head or hose, but even without those attached the flow is very weak. I can't access much more than where the hose attaches as it is behind the tiles, but is there anything I can do to figure out the issue and potentially resolve it?
Something could have broken in the shower valve. Most serviceable parts can be removed without having access to the wall. Each valve is different, but the normal steps are to remove the handle from the valve and then unscrew the valve core, or remove a clip and pull it out, or unscrew a collar, etc. You'll have to turn off the water to the house to work on this valve because there's no way to turn off the water just to the shower.
I've had this exact scenario when a damaged rubber washer tore in two and clogged the internal workings of the valve.
Most probably limestone rock or a piece of something coming with mains water clogged your shower valve.
But first check for economizer (is installed) just before the shower head, they tend to get clogged quickly and cost usually less than 5€.