The DHCP server may fail to start if the database becomes corrupt. An indication is an event in the Event Log like this:

Event ID: 1014
Source: DhcpServer
Type: Error
Description: The Jet database returned the following Error: -1022

To fix it, perform the following steps:

1. Stop the DHCP Service with net stop dhcpserver
2. Replace the DB file at %SYSTEMROOT%\system32\dhcp\dhcp.mdb with its backup from %SYSTEMROOT%\system32\dhcp\backup\jet\new\dhcp.mdb
3. Restart the DHCP server with net start dhcpserver

If this doesn't work, you may need to delete the entire %SYSTEMROOT%\system32\dhcp folder.