I always obey what it says on the strut - words to the effect of "Do not attempt to open..."
Around here, we have a mobile gas strut specialist (or two). They come around in a van and remove the strut for you, then they place it in a sealed chamber which they pressurise with nitrogen. This forces nitrogen into the strut, re-charging it.
I imagine that if the strut is completely 'flat', it may indicate that the seals have failed suddenly. In this case, the specialists can provide new struts - they are fairly universal - and expensive.
So generally what I have done in the past, with a range of vehicles, is to obtain a few struts from wreckers etc. and then I pick the best two. If these are not quite strong enough to do the job, I then call in the gas strut specialists and they recharge them for about US$30 the pair.
It's surprising how strong the strut still is once you remove it from the bonnet. Conversely it's surprising how heavy the bonnet is when you have to hold it up with one hand...