You typically would have a berthing licence (to include dry storage when the boat is ashore) and that gives you a right of access to your boat. If the yard has stated that you cannot access your boat then they have potentially revoked your licence - the reason is irrelevant; unless they have a force majeure clause or other clause that allows them to revoke access they likely cannot charge you in those circumstances - it’s very questionable (in your favour.)
Note that most savvy commercial landlords did not state that people could not enter their offices - they posed sensible precautions, and in many cases people did not attend the offices anyway, but they did not state that they couldn’t, precisely to avoid the above scenario.
Ardrossan have charged me £378 for 2 months ashore pro rata of winter rate. My email to them got lost . My invoice was sent out last Tuesday. As above give them a ring Simon Lamb I think answered my call. When they are allowed they will continue launching from when they left off.
Ours is still on it's winter trot mooring at Porthmadog, I haven't rung them yet to find out what they're doing about the situation, as the boat was supposed to be moved at the end of March. We're still hoping it'll be worth moving it to Abersoch for some part of the season. Fingers crossed!