There can be many reasons for your lack of access to your gmail account. It can be username, password or hacking issues. Date and Time Settings of the system can be another reason too, which we are discussing here.
Date and Time Settings of the system: If you have enabled secured browsing, the security certificate of websites will report error if your date or time settings of the system is changed to some previous date value. And hence you will not be allowed access. The same problem might prevent you from accessing even your facebook account too.
Change the date and time settings of your system(pc/laptop) to present value, you may even synchronize it with internet time and then try after refreshing; your gmail access issue might be resolved by now.