A flashing "A TRG'D SWP" indicates that there is a failure in the Kernel (pg 6-12, "Kernel Tests"). This means that the processor has detected a problem with its own operation, RAM, or ROM. This is before it gets to any self checks on scope-related peripherals.
If you look at tables 6-5, 6-6, and 6-7, it will try to tell you where it has found a problem. It does this by lighting some combination of Ch1 through Ch4 LEDs, "+" LED, "-" LED, and "READY" LED.
According to the tables, if nothing else is on besides the blinking "A TRG'D SWP", it indicates a probable issue with the A5 control board RAM or associated circuitry.
The manual also says you can press the "A/B TRIG" button to force the scope to run anyway. This may or may not work depending on the severity of the problem. If you do this and it starts working, you still need to figure out what's causing the kernel error since it may crash or not operate properly. Any other issues you might see at this point should not be considered a valid problem until you fix the kernel error.