If you are attempting to fly above roughly 175m, then you are likely to run into troubles. How do you know how high you are flying?
The top of your Second Life window shows your land location and your co-ordinates as 3 numbers separated by commas as shown above. The last digit represents your height from the ground which in the example screenshot above is 66 metres.
You will find that once you reach around 175m high, you no longer ascend despite holding your Page Up key. If this is you, you may wish to watch this youtube video for an explanation of what is happening.
Use the search function to find yourself some free flight feathers or other free objects such as jetpacks that you can buy.