You will see Event ID 3055 and 10006 on your Exchange cluster node. In Event Id 10006, you will also see this error message: 80070002 – The system cannot find the file specified..
When this occurred to me I simply restored the search service files to the original location and that resolved my issue. I was able to make the search service online again.
For rest of the details follow this MSKB: http://support.microsoft.com/kb/310586
Anand, the Architect 