Your best bet would be to get a consultant - or the vendor involved to help review the system.
For us to be able to help you we would need much more information:
1) What error are you getting when trying to start SQL Server (check application/system event logs - check SQL Server logs)
2) What changes have been made recently to your environment, server, SQL Server, etc...
3) Do you have a SAN - if so, is it up and available and all volumes presented and accessible?
Any other information you can provide to help identify what happened?