If an employee's laptop is taken back from him/her for purposes of investigation, can the IT department/team that is in charge of issuing laptops etc. log into the employee's network account on this laptop on their own or does the employee need to provide his/her credentials
This question is not actually specific enough to answer definitively.
Are you asking if they will be able to login to Windows on the computer using a domain account? (Yes, they can just reset your password)
Are you asking if they can access information contained on the laptop? (Also yes, unless there was encryption that you configured and only know the password to).
Are you asking about some other "network account"? (Still probably yes, businesses like to be able to manage employee access to products like this, which often involves resetting passwords)
As stated, for 'purposes of investigation' would indicate that anything and everything done on the employer-owned device will be thoroughly investigated. The IT dept. will be able to reset your password and gain access, or, depending on the level of investigation use forensics tools or a forensic company to gain the information they seek.