CS5 is fairly old - 9 years old to be precise - and Adobe doesn't support their older software anymore. In fact, there was a huge change done just two months ago regarding download links for older versions. Read more about it below:
» Adobe Limits Software Version Downloads, Incl. Some Direct Links - ProDesignTools
If you still have the DVDs for CS5, buy an external DVD drive. They are inexpensive and may be handy for future situations.
If you don't have the DVDs anymore, then upgrading to the Creative Cloud (see all plans) might be the best option.