What could be possible reasons on why all statements executing against a table would run extremely slowly causing blocking. No particular query was the culprit. At some point whatever was causing it ended and all statements started executing normal and all blocking was cleared up.
A corrupt index could cause the issue. If there are indexes, you can recreate them. If you're using table replication, if the replication is out of sync, this can cause slow queries especially if the tables handle a high volume of transactions. If you haven't done so, you may want to log the slow queries as even queries that take .5 seconds can quickly cause a bottleneck on high-traffic systems. Those are my "surface" thoughts. Other considerations such as disk space, RAM, disk integrity, etc. also come to mind. You may want to consider checking your system logs to see if anything shows up there during the time you experienced the issue.