Cannot redeem voucher with supplier

I purchased a meal voucher at the Il Tartufo restaurant from Groupon. The terms and conditions of the voucher state that only email reservations would be accepted. I emailed Il Tartufo on the 9th of May hoping to get a reservation, however I was told that they are fully booked that night and also the following weekend.<br> I tried again on the 13th of May. the 15th of May and again on the 18th of May to make a reservation (giving a range of available dates) however the restaurant does not respond at all to my mails.<br> The voucher expires on the 25th May and I am not able to book with the restaurant. <br> I would like a refund for my purchase if I cannot use the voucher because the restaurant is not accepting reservations.<br>