Fixing unresolved external symbol error in C++ - c++

Question

When I try to build my C++ application, I get a lot of unresolved external errors:
1>game.obj : error LNK2019: unresolved external symbol "public: __thiscall Player::Player(void)" (??0Player##QAE#XZ) referenced in function "public: __thiscall Game::Game(void)" (??0Game##QAE#XZ)
1>game.obj : error LNK2019: unresolved external symbol "public: __thiscall Player::Player(class std::basic_string<char,struct std::char_traits<char>,class std::allocator<char> >)" (??0Player##QAE#V?$basic_string#DU?$char_traits#D#std##V?$allocator#D#2##std###Z) referenced in function "public: void __thiscall Game::newMonster(void)" (?newMonster#Game##QAEXXZ)
1>game.obj : error LNK2019: unresolved external symbol "public: bool __thiscall Player::gameOver(void)" (?gameOver#Player##QAE_NXZ) referenced in function "public: void __thiscall Game::defaultMenu(void)" (?defaultMenu#Game##QAEXXZ)
1>game.obj : error LNK2019: unresolved external symbol "public: void __thiscall Player::updatePet(int,class Pet *)" (?updatePet#Player##QAEXHPAVPet###Z) referenced in function "public: void __thiscall Game::inGameMenu(void)" (?inGameMenu#Game##QAEXXZ)
1>game.obj : error LNK2019: unresolved external symbol "public: class std::basic_string<char,struct std::char_traits<char>,class std::allocator<char> > __thiscall Player::enemyName(void)" (?enemyName#Player##QAE?AV?$basic_string#DU?$char_traits#D#std##V?$allocator#D#2##std##XZ) referenced in function "public: void __thiscall Game::inGameMenu(void)" (?inGameMenu#Game##QAEXXZ)
1>game.obj : error LNK2019: unresolved external symbol "public: class Pet * __thiscall Player::getMonster(int)" (?getMonster#Player##QAEPAVPet##H#Z) referenced in function "public: void __thiscall Game::inGameMenu(void)" (?inGameMenu#Game##QAEXXZ)
1>game.obj : error LNK2019: unresolved external symbol "public: int __thiscall Player::getMonsterSize(void)" (?getMonsterSize#Player##QAEHXZ) referenced in function "public: void __thiscall Game::inGameMenu(void)" (?inGameMenu#Game##QAEXXZ)
1>game.obj : error LNK2019: unresolved external symbol "public: class std::basic_string<char,struct std::char_traits<char>,class std::allocator<char> > __thiscall Player::getName(void)" (?getName#Player##QAE?AV?$basic_string#DU?$char_traits#D#std##V?$allocator#D#2##std##XZ) referenced in function "public: void __thiscall Game::saveMonster(void)" (?saveMonster#Game##QAEXXZ)
1>game.obj : error LNK2019: unresolved external symbol "public: void __thiscall Player::addMonster(class Pet *)" (?addMonster#Player##QAEXPAVPet###Z) referenced in function "public: void __thiscall Game::loadMonster(void)" (?loadMonster#Game##QAEXXZ)
1>game.obj : error LNK2019: unresolved external symbol "public: void __thiscall Player::setName(class std::basic_string<char,struct std::char_traits<char>,class std::allocator<char> >)" (?setName#Player##QAEXV?$basic_string#DU?$char_traits#D#std##V?$allocator#D#2##std###Z) referenced in function "public: void __thiscall Game::loadMonster(void)" (?loadMonster#Game##QAEXXZ)
1>fatal error LNK1120: 10 unresolved externals
The thing is, these errors didn't show up a couple days ago. They just appeared suddenly out of nowhere.
I also have included my Player.h inside the Player.cpp and Game.cpp where it should be needed.
Can anyone help me here?