I booted up VS2010 today, after working all day without issues yesterday, to find that the editor no longer underlines any type of errors in VB.NET.
I also noticed that, under Tools > Options > Text Editor, I no longer have VB.Net option.
Yesterday, I imported "Son of Obsidian" style into my editor, which I've always used, and worked fine yesterday. This is the first time use after rebooting.
First,I suggest to reset vs2010 setting:
Go to Tools -> Import and Export Setting -> Reset All setting.
Second,
Reboot you VS2010 and load the solution. see if problem solved.
If yes, it means something in the "Son of Obsidian" style doing the issue - I suggests to download it again (maybe corrupted somehow)
If not, Import it again, reboot and see if problem returns.
Let me know if that solved your issue.