What else was going on on the server at that time?
- Is blocking occuring?
- Is another process monopolising resources (memory, cpu, i/o)
What has changed?
- Any code changes
- Any configuration change
- Any change in data volume (influx of new data)
Is all maintenance happening? (it won't take long to check)
- Are statistics up to date
- How is index fragmentation
- Are backups occurring (remember full backups control log fie growth)
Can the query be run manually?
- Look at Execution plan
Is it a stored procedure?
- Look at Execution plan for parameter differences (parameter sniffing)