Lock held by in-doubt distributed transaction n.n.nnnn

The title is self explanatory and I'll get quick to the solution:

select LOCAL_TRAN_ID,GLOBAL_ORACLE_ID,state,RECO_TIME ,top_db_user from PENDING_TRANS$;

Normally, you'll have to see the n.n.nnnn number under the local_tran column

rollback force 'n.n.nnnn';    (replace n.n.nnnn)