It's possible the files got corrupted during the transfer, although Windows is usually quite good at recognising when that happens and gives you a warning. But then if the system was freezing and you were getting errors......
I'm not sure on anyway to recover damaged files at this point without referring back to the old external drive.
If it is still just about working then plug it back in and run chkdsk on it. This may repair enough of it to transfer the files off of it again.
If you've got your new internal drive plugged into a SATA port then all is well. It should really be formatted in NTFS.