Since I had received your letter I couldn't open my computer. Is this sentence correct grammatically?
I think you probably need to say:
Since I received your letter, I haven't been able to turn on my computer.
Since receiving your letter, I haven't been able to turn on my computer.
I've changed "open" to "turn on" because we don't say "open a computer" in English. You can "open a file", or "open your email", but not the computer itself (unless you need to look inside it).