Our 6 year old dog having problems urinating. She had a bowel movement but has only peed once in 6 hours. Tries to pee but nothing comes out. Drinking a little water.

Does she seem to be in distress, moving all around to different spots? Are her legs trembling (dogs in pain don't always whimper). Are you sure she is trying to pee, or could she be trying to have another bowel movement?
6 hours just isn't enough time to get an idea of what's going on. Is it very hot where you are?
Kidney stones come to mind... make a vet appt in the morning.

What kind of dog is she
Please take her to the vet in the morning for urine and x-rays. A urine test will rule out infection which can cause these symptoms - but sometimes dogs with stones in their bladder and/or urethra occasionally will have no infection in their urine, therefore it's important to do both because stones can be easy to miss.