changing hosting companies should not matter! My advise is just to wait and see if your pages get picked up soon, if you they don't then you have to get in touch with the host and find out what they have done when they changed some of your pages? Maybe they messed with the coding? Good luck, if you need any help do not hesitate to ask!!!
I think IP address doesn't matter for SERP, try to get some more good quality back links so that your site will be again in rankings. all the best.
Sometimes this can happen if you lose a few vital backlinks too. If a site drops its own glamour with Google and it has backlinks to you it can hurt you in the SERPs a little, but I agree that getting some good one way permanent backlinks will have you at the top again in no time.