You have a couple of options, if I'm understanding you correctly. To access a variable from another class, you either need to pass it as a parameter or make it a class property. Broots Waymb's duplicate question link explains class properties.
To pass as a parameter would look something like this.
public class Form2 {
//Form2 class constructor
public Form2(string vInputFromForm1) {
//your code using passed in string
}
}
public class Form1 {
//...
public void someMethod() {
string vInput = textbox1.Text;
Form2 form2 = new Form2(vInput);
}
}
If neither of these options solves your problem, you may want to edit your question to add more information. As Dour High Arch mentioned, providing error messages can go a long way towards understanding the problem in the first place.