Do you have a BG-E2 or E2N grip on it? If so, check that it's snug. My 30D and 50D both used to stop unexpectedly - I mean go completely dead - if the grip loosened even slightly.
I also accidentally turned off the power on the grip accidentally more than once. (I've never really figured out why the grip needed a separate power switch.)
Even if it appears to be turned on, rotate that switch (if using a grip) and/or operate the off-on switch on the camera a few times. Those switches can get oxidization in them, but are self-cleaning if they are operated occasionally.
Besides that, I'd suspect your battery or charger. Do you have mulitple batteries, so that you can try more than one? If memory serves, the BP511/511A battery packs are good for about 400 recharge cycles, before they wear out and need replacement. I've also had those batteries not take a full charge the first few times, when they've sat unused for a long time and drained completely. At first, the camera would power up, but then just die almost immediately. After a couple charges I'd get one or two shots with it, then it would die again. Finally after maybe 6 or a dozen charge cycles the batteries started to take a full charge and the camera would work normally.
If multiple batteries fail to work, I'd suspect the charger. I've seen chargers go bad on rare occasion, too. Do you have a spare or can you borrow a known good one to try?
If the camera is fresh back from repair, if it isn't a simple problem such as these suggestions, I'd contact Canon about having them look at it again. I believe they warranty their repairs for some period of time, and since it's only been back a couple weeks I'd think it would be covered.
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Alan Myers
San Jose, Calif., USA
"Walk softly and carry a big lens."
GEAR: 5DII, 7D(x2), 50D(x3), some other cameras, various lenses & accessories
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