Why can't I change the quality of videos on YouTube?

When I click on a video, it begins to play in 360p. I click on the button to change to a higher quality-720p for example. When I do, the video screen freezes but I can still hear the audio playing. This began happening today and I have never had this problem before. I have an Acer Aspire with Windows 7 and I bought it in 2010. Please help!