UART doesn't work

I have an issue with an Arduino UNO board.
I cannot upload sketch using USB (i.e. UART).I don't know how it happened, but initially it was working quite well.
I tried to upload a sketch using SPI and it works perfectly. I also have changed a microcontroller with no result.
I tried to use USB connection on other computer as well.
I have another Arduino UNO board and it works with USB (so it seems that my computer works well).
One thing that might be a clue (but I'm not sure):
I uploaded a simple sketch which sends a "Hello" message through UART, and the terminal shows only "He".
Any thoughts about my issue?

If you have uploaded a sketch via SPI, the bootloader has been erased - you need to connect the ISP programmer again, select Arduino Uno as the board, and then choose Tools -> Burn Bootloader to put the bootloader back.

Also, be sure to disconnect anything you have that might be connected to pins 0, 1 or reset when uploading via serial - those pins are used for uploading, and external devices connected to them will interfere with uploads.

Thanks a lot DrAzzy! It works.
To be honest, I didn't know about that.
As to issue with "Hello" message - I've found out the problem.

As to issue with “Hello” message - I’ve found out the problem.

It’s always nice to share what the problem was so others can learn from it.

It's always nice to share what the problem was so others can learn from it.

It was pretty simple: I forgot to add sei() command so there were no interruptions (UART needs them).