Welcome, I can't help you ... even though I own a couple p95s and they are all dockers I've never broken one down past the factory recommendations... slide, & barrel. There has to be some kind of spring that goes in the back of the slide that causes the decock lever to flip back up to the horizontal.... take it apart and put it back together.... I'm thinking stoves pages or maybe the rugger site has the original operators manual which should have an exploded view off all the parts... sometimes these don't really show the exact position a spring needs to be in though.
I took the slide off of mine and there are two flat spring bars on one side and the upper one has a bend in it and seems to be the spring that causes the decock lever to return to horizontal.... now understand I don't have a clue how you took these things apart, I've never attempted to. Not sure I could take a decent picture to help further... but I'm sure someone else here can.
You can find the exploded view here: it really does not show you have to take all that apart, but as I mentioned I guess you have that figure out:
http://stevespages.com/pdf/ruger_p95_decocker.pdf