I did the main quest in the soul cairn and was then doing the misc.stuff and somehow i lost serena....i went threw the portal that put me back into the ruins but serena wasnt there....am i screwed?

Serana will usually reappear after a Cell Transition, She may just need time to catch up to Your character. She will often show up on the Volikhar Castle Balcony if She does not follow My character out of the Soul Cairn.

Try fast travelling somewhere.

Not sure if this applies, but if you tell her to wait, it can potentially trigger a bug where she will refuse to follow you. This issue was fixed by the Unofficial Dawnguard Patch (if you're on PC). If asking her to wait wasn't the issue then fast traveling (as others have mentioned) may solve it. Otherwise, you will probably need to reload a save prior to losing her.

fast traveling solved it...thankfully.

Glad it worked. I've had her remain lost after fast travelling before only to find her hanging around just inside the entrance to the last cave we visited together.