C# cannot convert database value to string

1. Try not to reuse connections unless you have to. The main reason is that you generally want to close connections as soon as possible and it will guard you against possible race conditions later if you have multiple events/actions that can occur at the same time that are data driven.
2. Close your connections as soon as possible so you do not have open external resources.
3. To ensure that 1 and 2 occur wrap your IDisposable types in a using block.
4. Always use parameters instead of string concatenation in your queries. It guards against sql injection (not applicable to MS Access) and ensures you never has issues with strings that contain escape charaters.
5. A note about MS Access and parameters: Place holders are usually specified by the ? character and are position dependent. This is critical, you cannot rely on the name of the parameter. If you have a parameter collection with 3 parameters in that collection then those parameters must appear in the same order in the query.
6. I notice you probably have password as plain text, never store passwords in plain text! Instead store a hash of the password. To see if the password matches the user's supplied password at login hash that input as well and then compare that hash to the stored hash to see if they are the same. Use a secure password hashing algorithm like pbkdf2, scrypt, or bcrypt.
7. To see if a row exists or to just return a single value you can also use ExecuteScalar with a null check as it will return null if no records are returned. I altered the code to just return accountnumber using ExecuteScalar. I also enclosed the column name in brackets which is good practice when including characters outside the range of a-z and 0-9 in your column name.
8. If you were to want to return data and read it using a data reader then do not use * for your return. Specify your column names instead. This will guard your code against schema changes like columns being added or column order changes.

Here is the updated code.

string accountnumber = null;

using(OleDbConnection connection = new OleDbConnection(/*add your connection string here*/))
using(OleDbCommand command = new OleDbCommand("select [Account_Number] from User_Info where Username = ? AND Password = ?", connection))
{
    command.Parameters.Add(new OleDbParameter("@username", OleDbType.VarChar)).Value = txt_Username.Text;
    command.Parameters.Add(new OleDbParameter("@password", OleDbType.VarChar)).Value = txt_Password.Text;

    connection.Open();
    accountnumber = command.ExecuteScalar() as string;
}

if (accountnumber != null)
{
    MessageBox.Show("Login Successful!", "Success!");
    this.Hide();
    User_Account_Screen UAS = new User_Account_Screen();
    UAS.Number = accountnumber;
    UAS.ShowDialog();
}