Talk to her and try to work it out. Do you have a rental agreement? Has she always paid on time? Is she in breach of any of the terms in the rental agreement? Do you plan on moving back in? Do you currently live there with her and only rent out a room? There is not enough information here to go by.
How long has she lived there? Less than a year, more than a year? That matters because of the amount of notice that you will have to give her.
Someone asked a great question earlier... Do you want to sell the unit?
This questions was also posted a while back so if you have an update for us that would be great. Let us know how it turned out.
All the best,
Coldwell Banker Star Realty
BRE Lic #01456982