If you are worried about your computer problems such as your hard disk crash, dead screens, computer lags, and any issues which render your Apple computer usable, you may get support from the Apple Genius Bar. If you have Apple Care for your computer, Apple will fix any problem with their product free of charge although it will not be a 100% guarantee that the issue will be resolve is not insured.
However, whenever you have a problem with your computer, try to perform some basic troubleshooting and try some workarounds first before you visit the Apple Genius Bar. You can read articles from Apple support website. This should be done as some problems can be fixed easily without bringing the product to the Apple Care and it much lesser time. Always remember to try restarting the computer as most of the time; the problem can be solved with a simple reboot.
If the problem will not be resolved by doing a simple reboot or by doing the steps in the Apple support website, then make an appointment with Apple Genius Bar. It can be made 24 hours ahead. You will find making appointment very easy because you can do it by calling local Apple store or signing up online.
You should bear in mind that making appointment as early as possible will be a good idea as they are catering service to a lot of Apple product users. Although it is not a guaranteed fix, they will surely try everything they can to fix the issue. Apple gives the service by priority basis. This is why I recommended making an appointment ahead of time as they follow first come, first serve policy. Apple has many customers which means not making an appointment equates into staying for hours in the waiting line.
Like any technical troubleshooting, be ready to describe in details how the issue is happening. You need to tell them how it started and how long has it been happening. This will help them identify the problem and provide resolution to it.