Can not dowload the code into my Altera

[ecenus]

I met a weird problem: I can download the code into my Altera chip EPM240T100 comfortably at the first time. But, I can not download the code again. The errors are:

Error: Can't access JTAG chain
Error: Operation failed.

I tried to do "Auto Detect", still an error window popped out as

Unable to scan device chain. Can't scan JTAG chain.

Why???

I appreciate your advice and tips!