Database Corruption & Repair
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.
Why Doesn't Microsoft Fix This?
Cynics would say that Microsoft benefits from this problem, as users are more likely to upgrade to SQL Server. This is almost certainly an exaggeration – the corruption problem is probably fundamental to the design of Access, and is unlikely to be fixed easily.