dont know why my script is causing unity to freeze.

very, very new to scripting in general, took 3 weeks of constant study to feel like i had a handle on it, turns out it wasnt long enough. can anyone tell me what im doing wrong? i understand the way my stuff is written is probably horrendous but like i said im new, and this is my first.

wanted a big block made of smaller blocks, it was a test. every time i run the code unity freezes, there is no compiling errors except it asking me if i meant x=x, which i do. (or dont depends on your answers :/)

 using System.Collections;
 using System.Collections.Generic;
 using UnityEngine;
 
 public class terraingen : MonoBehaviour {
 
     public GameObject gameobject;
     public float gridx = 0f;
     public float gridy = 0f;
     public float level = 0f;
     public float spacing = 0f;
 
 
     public void Start()
     {
         float retx = 0f;
         float retz = 0f;
 
         for (float y = 0f; y < gridy; y = y + spacing)
         {
             for (float x = retx; x < gridx; x = x)
             {
                 Vector3 pos = new Vector3(x, retz, y) * spacing;
                 Instantiate(gameobject, pos, Quaternion.identity);
                 
                if (y == gridy)
                 {
                     retx = retx + spacing;
                     y = 0f;
                     
                 }
                else if (retx == gridx)
                 {
                     retz = retz + spacing;
                     retx = 0f;
                 }
                else if (retz > level)
                 {
                     break;
                 }
             }
         }
 
     }
 
 }