How should I convert/sharpen a high resolution picture from a cropped image?

If anyone would solve this problem of mine, I'd cropped an image which is too blurry and pixelized and I just want that image to become a high resolution picture especially when that cropped image/picture will be displayed and printed on a large scale tarpulin (about 15x30 ft in dimension) but that issue is a huge problem and I'd read the other message board that said it's impossible to convert a low res picture or a cropped picture with low quality image to become a high resolution.

To get this straight: You cannot invent image detail where there is no.
So following this rule it's not possible to get higher resolution out of low-res images magically.

If you print it low-res you will get pixelation effect probably, but one thing you can do is upscale the image which used an interpolation technique to add missing detail, but this will then look kind of blurry.

Always ask for higher resolution images to be provided from the client. 99% of the time they won't have it, but then you explain that the resolution is insufficient for this job and ask him to take the risk of getting low quality printing results. It's the only way to play safe.
Good luck!