This is something frequently experienced by Android users. People tend to panic and think that there is nothing else they can do and they will not be able to download anything again. Calm down, we have 2 ways to fix this.
The first option -and the most recognized- is to clear the cache of Google Play. How can you do it? Go to Settings - Applications - Open Google Play Store - Clear the cache - Restart your phone.
The second way to fix this problem is to clear the history of Google Play Store. This is simpler than the first solution. First, you need to open Google Play Store - Settings - Clear History.