You can try playing via full tilt client. It uses the same log in information as your Pokerstars.
You can try Uninstall and reinstall Pokerstars Client.
Uninstall client then delete the
App data folder for Pokerstars. To find folder use "run" feature then type appdata in run box. It should take you to app data folder of your user. Check under local or roaming folders it should have a Pokerstars folder in one of them.
Note you may want to keep your current configurations like bet sliders themes etc and also hand and tournament histories. In the Pokerstars folder there move the hand history and tournament summary folders to another location as well as the user.ini file that contains all of your current settings. The hand history and tournament summary folder you want to move has your username at end.
Delete everything after moving files/folders.
Then download and install a fresh copy of Pokerstars client from the website. See if it would allow you to log in.
Go back to app data folder there will be new folders and files. Move back the hand history and tournament summary folders or just copy the contents of the old one to the new ones as well as the user.ini file.
Here is a picture of what my Pokerstars app data folder has in it.