Aspire R5-571T will not reconnect to ROKU tv through wifi

After much effort I was able to connect my Acer to the computer via wifi. Then at a later time when I wanted to watch a youtube presentation I was again able to connect. The next time, it would not connect and I had to use an HDMI cable. Since, after much effort, I still cannot connect by wifi my computer to our ROKU TV. I have spent more than 20 hours trying everything I know without success. I need help! TV is 2 years old and the model of the computer speaks for itself. The fact that I had it connected more than once and now it won't connect is puzzling.