The aim is to restart normal service at around 0500 if further testing proves OK.
This incident has involved a loss of signalling from (roughly) Farringdon to Loughborough Jn, due to a messy power failure to the signalling caused by some unusual combinations of tripping in the backup power systems. As yet I'm not sure anyone knows the exact cause.
Obviously the fact that this has taken place on a Sunday, with limited capacity south of the Thames for TL trains anyway (due to other engineering work), has meant that impacts have been somewhat limited. IIRC only one train was stranded away from a station and that seemed to only be relatively briefly. Had a similar failure occurred about 20 hours later (ie. Monday morning), and/or with next year's full TL service thrown in... Chaos, organised or not!