applied I-751 but not received renewed GC

[kartik01]

I had applied for removal of conditions (I-751 form) one year ago. few months after submitting I-751, I received a letter stating that they are having delay in processing of GC and that the letter can be used for travel and employment for 1 year.
Now that extension letter is also about to expire and I haven't received my new GC. I have to travel quite frequently to Europe for my work. Could you please tell me what should I do if the extension letter expires and I have travel out of US.

Thanks
Kartik