The only real reason why your hood won't stay up is because of the hood lift damper. A tell tale sign of this would be if some of the valve oil has leaked out of one of the ends. It could be old or leaking, but regardless of its reason for failure it needs to be replaced. You can't keep relying on your hood prop rod to keep the hood up. And if you are one of the unlucky few to have a vehicle without a hood prop, then a broken hood lift damper will force you to reply on a makeshift support like a 2x4 piece of lumber or your own arm... but that gets tiring after a while, and is extremely inefficient if you have to perform any work on the car that would require both hands. While this may just seem like a minor inconvenience, it can actually cause problems when taking your vehicle in for service. A mechanic may refuse to work on your car if they have to keep the hood up with one of their arms or if they have to rig up some sort of support system.
Hood support struts started replacing hood hinges on compact cars as early as the 1960s. They are now more widely used on vehicles made today than hinges are. Essentially, this little device is a small monotube shock absorber that installs at the very base of your hood. It connects to the firewall or fender, and has a fixed extended length. When your hood closed, the hood lift damper compresses the oil inside of it and keeps a pocket of gas under pressure. When the hood opens, that pressure is released and the struts push the hood up. They are rated at a pressure high enough to keep the hood up when they are fully extended, so there is a bit of geometry and physics involved in their design.
Still, hood lift struts are simple to install. They typically employ a ball and socket attachment at one end and will screw in at another. For a greater degree of mobility, ball and sockets will be on both ends. Most of the time the ball studs will be screwed into the hood and the fender, and the sockets at each end of the strut will clip onto them. It is recommended you support your hood securely with at least two methods of support, and you do it on the side opposite from the hood damper you are installing.
Universal lift supports are a great way to replace your broken hood lift damper, but will sometimes require a bit more work since you will have to use their kit to make the appropriate attachment hardware. These replacements are often cheaper, but are just as good as your original equipment. At Car Parts Discount, you will find excellent hood lift supports from Monroe, a top quality aftermarket brand. You can also find genuine and original equipment manufacturers like Stabilus and Sachs, but they will likely be more expensive. Either way, no matter what your preference is, CPD has you covered with great parts and super fast shipping. So no longer will you have to worry about your hood coming down on you any more.