The question is whether something has happened to block proper use of the lock file. Like perhaps a move of the location to a different folder with different properties, or a change of permissions. When the lock file (.LDB) cannot be properly manipulated, Access retreats to a "safe" option of treating the file as read-only.
A sql server table can prevent inserts and updates very commonly if something has a lock on it, which can happen for many reasons. For example, someone might be running a process with a strict isolation level. This is not a 'problem', it's by design.
In some cases you might not want to be able to insert or update records while someone else was trying to perform a read.