SRAM X4 MTB Rear Derailleur H-Limit screw maximum open, but still on 7th Cog? I own a 6 months old Rockrider 5.2 mountain bike. 

Everything was great until recently I noticed that the Rear Derailleur of my bike doesn't reach the highest gear cog, which is 8th. Its always stuck on 7th. 

I tried opening the H-Limit screw completely and it even came off, but my chain doesn't still move to the highest cog. 

I have a beautiful Sunday morning ride planned in 6 hours from now. Can anybody help?
First thing to check is if your rear derailleur is bent. If that is not the case, check the cable tension is not too high. Is the shifter acting funny (like hard to shift)?

See this link to see how to do adjustments: http://www.parktool.com/blog/repair-help/rear-derailler-adjustments-derailleur