I've done similar, but scotch-taped a hand written address 'label' over the name in the middle of the envelope, then put my return address in upper left corner.
Check your bank account statement to see if the check has been cashed. I am not suggesting that the person you sent it to you got it, cashed it, and is now lying to you. That would be beyond stupid, and quilters are way too smart to be that stupid! See my tale of woe, below.
I had a $900 check to my daughter go missing from the mail a few years ago. I believe it was stolen from her mailbox. (Not unheard of - after all someone stole my mailbox, post and all!) Whoever stole it took the check to the bank, forged my daughters signature on the back and cashed it. I got a copy of the canceled check: I have to tell you that the handwriting of the forged signature looked just like the handwriting of the teller who processed the transaction.
I did get my money back, but it took a while for the bank to return it to me. They had to 'investigate' the theft. I never heard whether they found the thief, but I did get the money back, so all ended well for me and my daughter.