while is your loop for incrementing progressbar value enable false the tabcontrol and when loop end than first show message box that process completed than enable true the tabcontrol.
it will work.
here is simple example:-
private void button1_Click(object sender, EventArgs e)
{
int Maxitem = 0;
tabControl1.Enabled = false;
Maxitem = 50000;
progressBar1.Maximum = Maxitem;
progressBar1.Value = 0;
for (int i = 0; i < Maxitem; i++)
progressBar1.Value += 1;
MessageBox.Show("completed");
progressBar1.Value = 0;
tabControl1.Enabled = true;
}