Try having everyone out of GP and running this in SQL:
DELETE DYNAMICS..ACTIVITY
DELETE DYNAMICS..SY00800
DELETE DYNAMICS..SY00801
DELETE TEMPDB..DEX_LOCK
DELETE TEMPDB..DEX_SESSION
When a user logs into GP, the ACTIVITY, DEX_LOCK, and DEX_SESSION tables creates a record indicating it.
When a user opens a transaction entry window, it creates a record in the SY00800.
Usually, when a user processes the transaction, it also creates a record SY00801.
These usually locks the transaction.
As long as the entry is not 'corrupted', you should be able to select it when you have deleted the above tables and get back into GP (and make sure no one selects that same JE).
Hope that helps.