First.. are you using a monitor with the correct scanrate?
The standard on an A2000 w/o a Display card is a PAL or NTSC 15MHz RGB.
An alternate is a composite (PAL or NTSC) attached to the Mono-RCA port beside the Audio RCA ports.
Second.. have you tried to boot off a floppy (such as a game)?
Third... have you tried to boot up in 68000 mode (possibly through holding the '0' key down at boot-time .... that's the one with the ')' on it)
What KickStart-OS is installed?