Pilot light not coming on

Question

I have ruud 90 plus achiever furnace about 11 years 0ld. the other day I noticed that the my house was getting cold and the furnace wasn't coming on. When I tried to reset it as per the instructions it didn't work until I turned the power and gas off and tuned back on 6 hours later in the morning that it worked. It worked for 18 hours and now same problem same solution again. How can I fix this problem?