Dump (examine memory before, at and after) 0x00000A5C and see what it decodes to.
Disassemble it using the debugger and then cross-check by hand-disassembling it as well.
Dump the whole vector area (from zero) and ensure the initial PC and SP are sane (and pointing to the above).
Can you single-step it from reset and see where it goes?
Internal or external FLASH?