There should be errors in the servers event logs. It may be as simple as the server drive being out of free space. Since the store seems to mount after a reboot, u may have exceeded the default maximum store size. Exchange 2003 has a default max store limit of 16GB. When you approach or exceed that limit, the store will unmount. If loaded Exchange 2003 SP2 u may modify the maximum store size up to 75GB. U could also perform an off-line defragmentation of the message store to reclaim empty space in the database.
If it is not a store size problem, it could be a damaged information store or corrupt transaction log file. U may link to this site to get more help :
Hope it helps u !