Without a database server, each copy of Access works directly on the database file. If there is some interruption while it’s working – such as a crash – then the program might not have written all the changes it was working on. The result is a database that contains errors, and cannot necessarily be opened. This is described in the Microsoft Knowledge base – see Article Q303519.
The likelihood of a database becoming corrupt depends on a number of factors: the number of users, the type of updates and the chance of an interruption. It can also depend on how the software writing to the database has been structured.