My Silky Terrier is having trouble standing on her back legs; this came on suddenly and I took her to be x-rayed and all was good. I nor the vet could find any injury or tense muscles. She is an inside dog and only gets on furniture when I put her there. She is 18 years old,but in very good health otherwise, 80% eyesight. Any ideas of what could cause this. It is now 24 hours later and she still cannot stand on her back legs

18 is a great long life for a terrier, usually life is 12-16. Has the vet brought this fact up to you? This is likely old age and nothiny can bring about a significant improvement

Wow 18 years old! He could have had a stroke, and with his age, that's what I would put my money on. After a stoke at his age, there isn't much chance of a full recovery. You could talk to your vet about giving him half an aspirin, but you would DEFINITELY want to talk to your vet about correct dosages.