Rent is late. Tenant not responding to email

[Douglas T.]

I have a parking space in Chicago that I rent out.  I have had the same tenant for 2+ years with no real problems.  Her rent was due on 12/15.  I waited 10 days until I contacted her via email.  I emailed her on the 26th, 28th, and 30th with no reply.  Maybe I'm overreacting since this has occurred in the time b/w Christmas and New Years.

She has paid her last month's rent up front so she's technically paid through Jan 15th.  I've sent her 3 emails with very cordial wording.  What should my next move be?  I wouldn't have any trouble finding a new renter in the Chicago winter.  When should I consider our lease terminated?