There can be multiple reasons for this error:
- Missing transaction log files
- Exchange Information Store unable to start
- Not enough free disk space on the database or log file volume
- Exchange “Dirty Shutdown”
- Corrupt Exchange Database Files
Also, check this detailed article for help, and let me know if you are able to solve the issue or not - How to resolve unable to mount database (hr=0x80004005, ec=1108)