The so-nicknamed Amorian (presumably for remarrying after his first wife died) came to power after rebelling against Leo V who had at one time been one of his closest friends. Their relationship soured on account of his own greed and, well, just because he could take advantage of his former friend's declining popularity.
Because of his betrayal he started off very unpopular himself and this, in turn, spawned a rebellion by a certain Thomas the Slav who gathered an army of soldiers sympathetic to the memory of Leo and they set Constantinople on siege. However, his siege was not well planned and ultimately failed. After suffering another defeat in open battlefield he fled to a fortified city and in so doing found himself playing the part of besieged to Michael's army. As conditions deteriorated for the holdouts the rebel leader was handed over to Michael in exchange for amnesty for everyone else and then summarily executed.
After the elimination of his chief rival Micheal went on to rule wisely until his peaceful death in 829 at which time power passed to his son Theophilus.
SB 1640, DOC III 5 AU Solidus Obv: MIXHLbSILS - Crowned facing bust of Michael, holding cross potent and akakia.
Rev: θOFILOSP+X - Crowned facing bust of Theophilus, holding globus cruciger and cruciform scepter. (Constantinople). $1,650 4/1/01.
SB 1642, DOC III 9 AE Follis Obv: IXHLSθEOFILOSbASI - Busts of Michael and Theophilus facing; cross above, dot between.
Rev: Large M, cross above, XXX left, NNN right; under M. (Constantinople).