Zone Compiling Error

[Elysius]

Today I was trying to make some changes to my old server code, specifically to add Attack Delay that was put in back in 2014 (like I said, old). I backed up my zone folder in case things didn't work out, which they didn't as I assume there was some other update required to make the new code work. So I figured no problem, I'll just go back to my original code.

But ever since I did this, I can't compile anymore. Even with my code back to the way it was before. I keep getting this error:

1>------ Build started: Project: ZERO_CHECK, Configuration: Debug Win32 ------
2>------ Build started: Project: zone, Configuration: Debug Win32 ------
2>horse.obj : error LNK2001: unresolved external symbol "public: virtual void __thiscall NPC::SetAttackTimer(void)" (?SetAttackTimer@NPC@@UAEXXZ)
2>merc.obj : error LNK2001: unresolved external symbol "public: virtual void __thiscall NPC::SetAttackTimer(void)" (?SetAttackTimer@NPC@@UAEXXZ)
2>npc.obj : error LNK2001: unresolved external symbol "public: virtual void __thiscall NPC::SetAttackTimer(void)" (?SetAttackTimer@NPC@@UAEXXZ)
2>pets.obj : error LNK2001: unresolved external symbol "public: virtual void __thiscall NPC::SetAttackTimer(void)" (?SetAttackTimer@NPC@@UAEXXZ)
2>C:\EQ\Source\Build\Bin\Debug\zone.exe : fatal error LNK1120: 1 unresolved externals
3>------ Build started: Project: ALL_BUILD, Configuration: Debug Win32 ------
3>  Build all projects
========== Build: 2 succeeded, 1 failed, 9 up-to-date, 0 skipped ==========

Which is strange because the SetAttackTimer was what was being editted. But I've since replaced those edits with my original backed up code.