TEMPDB can fill up and lock users/queries if you don't have it set to automatically grow enough. Notice I said 'enough'. The growth size has to be enough that it can grow faster than the space is filled up. For example, lets say you set it to auto grow by 100 MB. You get a bunch of transactions in a very short period of time and the transactions equal 150 MB of data. If the database can't auto grow fast enough (in this case twice), it will lock up.

How do you have it set to grow? By percentage or by a set size? How much is it set to grow by each time?

-SQLBill