Champagne not stored properly
Unless its an older bottle your chances are pretty good that it will be ok. However, sparking wine is tricky because if the cork degrades it will lose its effervescence. If you are concerned, send it to me & I'll let you know if t was good or not. Seriously, I'd drink it ASAP and if it's a special bottle for a celebration I'd open it first and have another bottle on hand in case the first one is bad.
it's not improper to store Champagen upright.
How long has it been stored? Unfortunately Champagne cooks do lose elasticity over time even when stored horizontally. % years you should not have any problems.
If cork has shrunk laying bottle on side won't repair the damage and may make things worse (as upright a layer of CO2 on the surface of the wine may help protect it.
Only way to be sure is to open and drink it.
(comments refer to Champagne and similar quality methode champenois wines)
"Sherry producers suggest storing their products upright, lest the heightened level of alcohol erode the plastic-capped cork. Madeira houses likewise insist their bottles should remain vertical, and Champagne may stand upright during long-term storage, a practice advocated by the CIVC."
source: Guild of Sommeliers