I could be wrong, but I suspect that your hard disk either has or is developing a problem.
In this case, make sure that you’re backed up! You don’t want to lose those pictures!
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What the messages mean
The whole “CBlockFile::Open Block” thing sounds like the program’s disk reading code has a problem. It’s trying to read something from the disk (that’s the Open Block part) and failing (that’s the error part that you’re seeing).
Now, I’m not sure what Google (the team behind Picasa) said, but ultimately, I’m concerned. If this is a hard disk issue, it’s actually good that the problem didn’t go away so that you’re not fooled into thinking that it was fixed.
Back up now!
That article tells you to run ChkDsk a couple of times with different switches, so that there are multiple levels of aggressive file checking and disk repair. Then, run another tool or two depending on how much you want to invest in recovering this hard disk.
If ChkDsk reports unrecoverable errors, then either you’re going to want to do something like run Spin Rite (which will cost you some money) or get a replacement hard drive (which will also cost you some money).
Now, I may be wrong, but that’s what this error message feels like to me. I’ve encountered this kind of cryptic message before in low-level disk reading software.1
The important thing to take away here is to back up now!