Most often when you get this message, it is an early warning that many errors have been detected and fixed already, and the drive WILL fail sometime soon. It usually does not mean that the drive already has failed and you have lost things. You MAY have lost a few files so far, but quite possibly you have not lost anything.
The proper next step is to stop using the computer for a short time (if possible) until you can get a new replacement drive. You also will need some Cloning software, so that you can Clone your old drive's contents to the new one. Many drive makers will give this to you for free on their website as an inducement to buy their HDD. For example, Seagate makes their Disk Wizard available for free download, and it will make a clone from ANY old HDD, but only TO a new Seagate drive. Similarly, WD has their Acronis True Image WD Edition for download, to make a clone to a WD drive. Other makers may have similar free tools. Or, you can pay for commercial software, or maybe find a free utility.
Many of these packages do a LOT more than just cloning, so make sure to get the user manual (usually part of the download) and read it.
You install the software on your current drive. Then you install the new HDD in your machine and run the cloning software. It will make a complete copy of EVERYTHING from old to new, putting all the special stuff exactly where it needs to be, so that the new one is completely able to take over in place of your old C: drive. As long as there are not big errors on the old HDD yet, this clone will work perfectly.
I've found one wrinkle in the way these cloners work that I think needs your intervention. This is why reading the manual, so you know how to navigate the menus, helps. By default, many of them will make the clone copy the SAME size as the old drive. But most often your new unit is larger, and you want ALL of its space included in the new C: drive to make it easy to use. So when it shows you its default plan for how to create the new clone, you need to NOT automatically say "Yes". You need to use the menus to alter the size of the new Partition it is creating to include all of the new unit's space. In most cases, all the other default choices are OK. THEN you let it proceed.
When the cloning operation is done, you shut down. Open up the case and disconnect the power and data cables to your OLD drive. Now go to the NEW drive and disconnect only its data cable, then connect to it instead the data cable that WAS for the old drive. In this way your new HDD will become connected to the same SATA port that your old drive was on. For now you can leave the old HDD in the case, still completely disconnected. Close up and boot, and it should all just work, except that your C: drive will probably have a new larger size. Easy, eh?
Now, that failing old HDD is still a perfect backup of your system before this all was done, right? You should not need it, but it's there just in case. AFTER you are completely satisfied that the new drive has everything working and you don't need the old one, remove the old HDD from the case and throw it out. Don't try to fix it and re-use - it will just fail later and cause you grief.